1. Field of the Invention
The present invention relates to a PM stepping motor with reduced noise.
2. Description of the Related Art
Claw-pole Permanent Magnet (PM) stepping motors have been widely used in OA equipment, computer peripheral equipment, air-conditioning equipment and so on. The claw-pole PM stepping motor includes a stator and a rotor having a cylindrical permanent magnet. The rotor is held inside the stator in a rotatable state. The stator has a structure in which two sets of stator units are joined to each other. Here, the stator unit includes an inner yoke, an outer yoke and a coil disposed therebetween. Further, the stator is configured by joining two stator units in an axially overlapped manner such that the inner yokes thereof are arranged in a back to back relationship.
Further, each of the inner yoke and the outer yoke includes a plurality of pole teeth extending in an axial direction. The pole teeth of the inner yoke and the pole teeth of the outer yoke are alternately engaged with each other with a space therebetween. The rotor is rotatably positioned inside these pole teeth via a predetermined air gap. For example, JP-A-H10-295070 discloses the PM stepping motor having the above-described structure.
FIGS. 6 to 8 show the PM stepping motor disclosed in JP-A-R10-295070. As shown in FIGS. 6 to 8, a stator 2 has a structure where two stator units are overlapped in an axial direction. The stator unit is configured by an inner yoke 3, an outer yoke 4 and a coil 5 disposed therebetween. Here, the two stator units are joined in a state where the inner yokes 3 thereof are arranged in a back to back relationship. The outer yoke 4 is a cylindrical member made of soft magnetic material and includes a bottom portion 41 and a cylindrical outer peripheral portion 42. A plurality of pole teeth 44 are formed on an edge of an opening 43 which is formed in the center of the bottom portion 41. A cutout 45 is formed in the outer peripheral portion 42. A terminal portion 53 formed on a bobbin 51 passes through the cutout 45 and protrudes to the outside thereof.
The inner yoke 3 is a disc-shaped member made of soft magnetic material. A plurality of pole teeth 34 are formed on an edge of an opening 33 which is formed in the center of a base portion 31. A cutout 35 is formed in an outer peripheral portion of the base portion 31. The terminal portion 53 formed on the bobbin 51 is intended to protrude to the outside through the cutout 35. Further, the base portion 31 is formed with a protruding portion 37 and a hole 38, which are located at positions facing each other via the opening 33. The protruding portion 37 and the hole 38 are respectively engaged with the hole 38 and the protruding portion 37 of the other inner yoke 3, so that positioning and joining between the inner yokes 3 are performed.
When the claw-pole PM stepping motor 1 disclosed in JP-A-H10-295070 is driven, the rotor 6 is vibrated in an axial direction at the switching of energization to the coil 5, which causes generation of noise. Further, simultaneously, the stator 2 is also vibrated in a radial direction by electromagnetic force acting between a permanent magnet 62 of the rotor 6 and the pole teeth 34, 44, which causes generation of noise.
Here, in the PM stepping motor 1 disclosed in JP-A-H10-295070, the positioning between the inner yokes 3 is performed by the protruding portion 37 and the hole 38, which are formed on the inner yokes 3. The protruding portion 37 and the hole 38 are respectively formed at a position of 90° with respect to the cutout 35 through which the terminal portion 53 passes and protrudes to the outside. In this structure, there is a problem that rattling is likely to occur in the holding state of the inner yokes 3 and noise is likely to increase due to the vibration at driving.